However, Carin Meier’s Living Clojure is excellent in many ways. Get it from O’Reilly (we’re an affiliate):

My little tutorial started with part zero, in which I lamented how functional programming is made to appear unlearnable by mere mortals, and it kind of snowballed from there. Hope you like it and/or find it useful!

In the last four installments of Functional programming without feeling stupid I’ve slowly built up a small utility called ucdump with Clojure. Experimentiing and developing with the Clojure REPL is fun, but now it’s time to give some structure to the utility. I’ll package it up as a Leiningen project and create a standalone JAR for executing with the Java runtime.

Creating a new project with Leiningen

You can use Leiningen to create a skeleton project quickly. In my project’s root directory, I’ll say:

lein new app ucdump

Leiningen will respond with:

Generating a project called ucdump based on the 'app' template.

The result is a directory called ucdump, which contains:

.gitignore README.md project.clj src/
LICENSE doc/ resources/ test/

For now I’m are most interested in the project file, project.clj, which is actually a Clojure source file, and the src directory, which is intended for the app’s actual source files.

Leiningen creates a directory called src/ucdump and seeds it with a core.clj file, but that’s not what actually what I want, for two reasons:

I want ucdump to be a good Clojure citizen, so I’m going to put it in a namespace
called com.coniferproductions.ucdump.

My Git repository for ucdump also contains the original Python version of the application, which is in <project-root>/python, and I want the Clojure version to live in <project-root>/clojure.

In my recent post Functional Programming Without Feeling Stupid I took a quick look at how functional programming can be a little off-putting for the non-initiated. I promised to provide some examples of my own first steps with FP, and now I would like to present some to you.

Advocates of functional programming often refer to increased programmer productivity. At least some of that can be attributed to the REPL, or the Read-Evaluate-Print Loop. We are basically talking about an environment which accepts and parses any code you type in, and gives you a place to experiment and see results quickly. Before interpreted or semi-interpreted languages like Python, Java and JavaScript became mainstream, the typical repeating cycle in software development was Compile-Link-Execute, and debugging meant observing special output on the console. In the 1990s integrated debuggers with watches and breakpoints became the norm, but long before that Lisp-like languages already had a REPL, and Python also acquired one.

If you are thinking about getting intimate with Clojure, you will need to get to know the REPL. It is your playground, and will always be, even if you later start packaging and organizing your code.

Clojure depends on the Java Virtual Machine (JVM) and is actually distributed as a normal JAR file (Java ARchive), like most Java libraries are. You can start Clojure from the JAR, but you will save yourself some trouble and prepare for the future if you install Leiningen, the dependency management tool for Clojure. It is simple to install and run, and I will assume that you will follow the instructions on the Leiningen web site sooner or later. Now would be a good time.

When you’re done with the installation, you only need to say

lein repl

to start a Clojure REPL. I’m using OS X, so what I describe here was done from Terminal. You don’t need to create a project with Leiningen if you just want to play around in the REPL.

Of course, if you don’t have Java installed, you need to get it first. Refer to the Java web site of Oracle for details as necessary. Furthermore, some of the things I will describe require Java version 7 or later.

If you follow software design trends (yes, they exist), you may have noticed an increasing amount of buzz about functional programming, and particularly the Clojure language. While functional programming is hard to define, almost everyone mentions pure functions, the lack of side effects and state, and easy parallelisation. As for Clojure, it is all about (a kind of) Lisp running on the Java Virtual Machine (and .NET, and transformed to JavaScript).

I’m somewhat convinced that functional programming is at least worth knowing about and trying out, even if you don’t expect to fully convert. It has been said that learning about the functional paradigm makes you a better programmer in your current imperative language. Functional languages reduce accidental complexity, and that helps you focus.

“Whoop de doo, what does it all mean, Basil?”

If you have a background in imperative languages, you will have an interesting time if and when you start digging into functional programming, because whatever else it is, it’s different. And I’m not talking about syntax only, but most of what you do. If you need to add an item to a list, you construct a new list with the new item appended to the previous list (no, it is not as inefficient as it sounds, because there is great stuff under the hood to handle that). This is because immutability is one of the cornerstones of functional programming. If you can’t change something after it is created, there is no state to mess up. You program with values, not stateful objects.

I see I’m getting myself tricked into presenting a definition of functional programming, when that has been done better elsewhere. For pointers, see Michael Fogus’ 10 Technical Papers Every Programmer Should Read (At Least Twice), including the classic “Why Functional Programming Matters” by John Hughes. But I actually wanted to talk about something else.